The invention is in the field of electronic reproduction technology and is directed to a method for positioning engraving elements in an electronic engraving machine for engraving printing cylinders for rotogravure, whereby at least two engraving lanes of predetermined lane widths lying next to one another in an axial direction on the printing cylinder are engraved with a respectively allocated engraving element, and is also directed to an electronic engraving machine and a position measuring unit for the implementation of the method.
When engraving printing cylinders in an electronic engraving machine, an engraving element that comprises an engraving stylus as cutting tool moves along a rotating printing cylinder in the axial direction either continuously or in steps. The engraving stylus controlled by an engraving control signal cuts a sequence of depressions, referred to below as cups, arranged in an engraving grid into the generated surface of the printing cylinder. The engraving control signal is formed from the superimposition of an engraving signal representing the gradations between xe2x80x9cblackxe2x80x9d and xe2x80x9cwhitexe2x80x9d with a periodic raster signal. Whereas the periodic raster signal effects a vibrating lifting motion of the engraving stylus, the engraving signal controls the depths of the cups engraved into the generated surface of the printing cylinder in conformity with the gradations to be reproduced.
For magazine printing, a plurality of stripe-shaped cylinder regions, called engraving lanes, that lie axially next to one another on a printing cylinder must be simultaneously engraved with a respective engraving element. For example, the various printed pages of a print job are engraved in the individual engraving lanes. The engraving elements allocated to the individual engraving lanes are mounted on an engraving carriage that moves along the printing cylinder in axial direction during the engraving.
A precondition for a good reproduction quality is registration-precise adherence to the lane widths of the individual engraving lanes in axial direction of the printing cylinder. In order to achieve a registration-exact engraving of the engraving lanes, the distances between the tips of the engraving styli of the individual engraving elements in the traditional method must be set in the axial direction of the printing cylinder by axial displacement of the engraving elements on the engraving carriage, being set with high precision to the required lane widths, and the engraving carriage with the engraving elements must then be displaced such relative to the printing cylinder such that the engraving stylus tips are positioned to the respective, axial start of engraving positions of the engraving lanes.
The traditional positioning of the engraving stylus tips of the engraving elements onto the lane widths essentially occurs manually by an operator, in that the latter initially roughly sets the axial spacings of the engraving elements from one another corresponding to the lane widths, and the engraving stylus tips of the engraving elements are then finely positioned upon visual observation of the engraving stylus tips with the assistance of a specific microscope unit (stylus allocation rule) and spindle drives actuated by hand.
This manual procedure is time-consuming, particularly when a great number of engraving lanes are to be engraved and, thus, a great number of engraving elements must be positioned. Moreover, the adjustment precision is essentially dependent on the care of the operator.
WO Published application 95/31332 already discloses a system for the automatic axial positioning of a plurality of engraving elements when engraving printing cylinders, with the assistance of motor drives for the individual engraving elements arranged on an engraving carriage and via a sensor monitoring.
An object of the present invention is to improve a method for positioning engraving elements in an electronic engraving machine for engraving printing cylinders for rotogravure as well as an electronic engraving machine for implementation of the method such that the adjustment of the axial spacings of the engraving elements relative to one another occurs automatically in a short time with high precision.
According to the method and apparatus of the invention for positioning engraving elements in an electronic engraving machine for engraving a printing cylinder for rotogravure, where at least two engraving lanes of predetermined lane widths lying next to one another in an axial direction of the printing cylinder are engraved in the form of cups with a respectively allocated engraving element, the engraving elements being positioned axially relative to the engraving cylinder before the engraving, with the engraving elements implementing an axial feed motion along the printing cylinder during engraving. Axial reference positions are predetermined for the engraving elements, axial spacings thereof from one another corresponding to the predetermined lane widths of the respective engraving lane. An electronic position measuring system is successfully positioned to the axial reference positions. An axial deviation of a reference point of the allocated engraving element from the reference position is identified with the position measuring system in every reference position. The engraving elements with their reference points are axially displaced by the identified deviations onto the corresponding reference positions.
In a position measuring system for an engraving machine for engraving cups on a printing cylinder with engraving elements, a video camera is mounted on a measuring carriage for registering video images of engraving stylus tips of the engraving elements or of cups engraved as trials with the engraving elements as axial positions of the engraving elements roughly shifted onto predetermined reference positions. The measuring carriage is displaceable in an axial direction of a printing cylinder and positionable to the predetermined reference positions for the engraving elements. An image evaluation unit for determining axial deviations of the actual positions of the engraving elements from the reference positions by interpreting the registered video images is provided.
The invention is explained in greater detail below with reference to the drawing figure.